1. Field of the Invention
The present invention relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which acquire k-space data in frequency space (k-space) in non-Cartesian state by rotating a zonary region, referred to blade, formed by plural parallel data acquisition loci by every TR (repetition time) and fill the k-space data in the k-space.
2. Description of the Related Art
Magnetic resonance imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF signal having the Larmor frequency magnetically and reconstructs an image based on a MR (magnetic resonance) signal generated due to the excitation.
As one of data acquisition methods in the field of magnetic resonance imaging, there is a method for acquiring and filling up k space data in the frequency region in non-Cartesian (nonorthogonal) state by rotating, at every repetition time, a zonal region called a blade formed by multiple parallel data acquisition loci (see, for example, “Multishot Diffusion-Weighted FSE Using PROPELLER MRI”, Pipe et al., Magnetic Resonance in Medicine 47:42-52(2002)). This method is sometimes referred to as PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) method or BLADE method. The method is henceforth called simply the blade rotating data acquisition method. And a pulse sequence according to the blade rotating data acquisition method is referred to as a blade rotation data acquisition sequence.
In the blade rotating data acquisition method, data is acquired in each blade by an imaging method, such as a FSE (fast spin echo) method, which can perform multi shots. Further, the blade rotating data acquisition method is a scanning method for rotating a blade around the origin of the k space as the center by changing gradient magnetic fields to form multiple blades radially around the origin of the k space as the center.
That is, in the blade rotation data acquisition sequence, in order to acquire pieces of data in different blades, a frequency encoding gradient magnetic field pulse showing mutually different patterns with respect to respective blades is applied. More specifically, with an absolute value intensity |Gf| of the frequency encoding gradient magnetic field pulse Gf maintained, only an application direction of the frequency encoding gradient magnetic field pulse Gf is changed by changing an intensity ratio Gx:Gy (Gf2=Gx2+Gy2=const) between an x-axis direction component Gx and an y-axis direction component and Gy of the frequency encoding gradient magnetic field pulse. Herewith data acquisition points within a blade in the k space (Fourier space) that is a spatial frequency region rotate on the same plane.
FIG. 1 is a diagram showing an example of data sampling using a conventional blade rotation data acquisition sequence.
FIG. 1 (a) shows an example of a frequency encoding gradient magnetic field pulse Gx in x axis direction. FIG. 1 (b) shows an example of a frequency encoding gradient magnetic field pulse Gy in y axis direction. FIG. 1 (c) shows data acquisition points in k-space by applying a frequency encoding gradient magnetic field pulse Gf having an intensity ratio shown in (a) and (b) of FIG. 1. The respective ordinate axes of (a) and (b) of FIG. 1 denote intensities of the frequency encoding gradient magnetic field pulse Gx which is x axis direction component and the frequency encoding gradient magnetic field pulse Gy which is y axis direction component respectively. The abscissa axes of (a) and (b) of FIG. 1 denote time. The abscissa axis of FIG. 1 (c) denotes frequency Kx in k-space corresponding to spatial position in x axis direction. The ordinate axis of FIG. 1 (c) denotes frequency Ky in k-space corresponding to spatial position in y axis direction.
As shown in FIG. 1 (a) and FIG. 1 (b), for example, when the intensity ratio between the x-axis direction component Gx and the y-axis direction component Gy of the frequency encoding gradient magnetic field pulse is equal, the data acquisition points in the k space as shown in FIG. 1 (c) lie in a straight line declining by an angle of 45 degrees from the frequency axes Kx and Ky corresponding to the x-axis and y-axis respectively.
Data is acquired by an imaging method that can perform multi shots as described above, and a FSE sequence is generally employed. In the normal FSE sequence, the number of data equivalent to an ETL (echo train length) is acquired per single excitation from data acquisition points on plural parallel lines in a same direction. Consequently, data acquisition locus equivalent to the ETL becomes a zonal area, and this zonal area configures one blade. In other words, data for one blade is acquired by one excitation.
Then, excitation is repeated by times equivalent to the number of shots and pieces of data within multiple blades are sequentially acquired. At this time, rotating the blade with respect to each excitation fills up data at all data acquisition points in the k space.
FIG. 2 is a diagram showing k-space filled with data acquired by the conventional blade rotation data acquisition sequence.
In FIG. 2, the abscissa axis denotes position Kro in the RO (readout) direction in k-space and the ordinate axis denotes position Kpe in the PE (phase encode) direction in k-space. The respective points in FIG. 2 denote data acquisition positions.
As shown in FIG. 2, for example, the x-axis direction is regarded as the RO direction and the y-axis direction is regarded as the PE direction. FIG. 2 shows an example in case where data is acquired from 6 blades having mutually different directions from blade No. 0 to blade No. 5 respectively. Data is acquired from data acquisition points on three parallel lines within each blade. This type of data acquisition from plural blades having mutually different directions fills up data at all data acquisition points in the k space ultimately.
Since data acquired by the foregoing blade rotation data acquisition sequence is influenced due to motion of an object that is a patient, correction processing is performed to eliminate the influence of the motion from data. Specifically, correction of a relative rotational and a translational amount is performed among pieces of data belonging to mutually different blades to eliminate the influence of the rotational and translational motion of the object among blades. This correction, for example, is performed by performing FT (Fourier transform) of data corresponding to each blade in the k space once, then by canceling a relative rotational movement and translational movement among respective images acquired corresponding to plural blades by correction, and subsequently by restoring the corrected data to data in the k space by the inverse FT. The correction of the relative rotational movement and translational movement among the respective images can be performed by rotating and/or moving the respective images so that the cross-correlation of each image obtained by correlation processing with a certain reference image increases (see, for example, “Effect of Motion Correction Associated with Echo Train Length and Number of Blades in PROPELLER MRI-Computer Simulation”, Isao Micro et al., JAPANESE JOURNAL OF RADIOLOGICAL TECHNOLOGY, Vol. 60 No. 2 pp. 264-269 FEBRUARY 2004).
In addition, weighting processing is performed to exclude data that is subjected to correction of a rotational amount and a translational amount but not appropriately corrected due to the influence of motion between slices and the nonrigid motion from a target of image reconstruction processing.
The foregoing motion correction is separately performed with respect to each slice. This means the motion correction of data is performed per slice, and performed per blade in each slice.
Since data after the motion correction in the K-space is data in the nonorthogonal coordinate system, it is converted into data in the Cartesian coordinate system by gridding processing. An image for displaying is reconstructed by FT of the data in the Cartesian coordinate system.
However, when performing the conventional motion correction on data acquired by the blade rotation data acquisition sequence, there is a problem that an artifact might occur in an image. This caused by the possibility that an error occurs in correction processing and a correction amount becomes an abnormal value depending on a motion correction algorithm and data to be corrected. That is, when correction of a relative rotational amount and translational amount or weighting processing among blades is performed using an abnormal correction amount acquired by correction processing, a remarkable artifact may be generated.
In particular, when performing a rotational correction on data showing a low sensitivity to a rotation like data acquired from a parietal region to obtain an axial image of head part, namely having a high isotropy to a rotation, errors occur and correction amounts become abnormal values frequently. In such a case, it is known that an artifact called Crinkling occurs.